This is a portrait of Memnoch the Devil as described by Anne Rice in the book of the same name. Memnoch in the book is an archangel who challenged God on so many fronts that God cast him out of Heaven and made him appear in a grotesque form. Memnoch was dubbed 'Satan' which means 'Challenger'.
In his demon form, he is supposed to look like he did as an Archangel: beautiful in feature and large in stature, but he is the colour of black granite from head to toe and has the addition of goat's legs to make him more ghastly and monstrous (which Lestat finds horrifying). If and when he returns to Heaven for a visit, he is supposed to make himself appear as an Archangel again - all white and beautiful etc. On Earth, in Hell and in Sheol he cannot appear as an Archangel - only as the goat-legged, black-as-granite demon being or as the 'Ordinary Man' for ease of conduct in the mortal world.
His purpose as the 'adversary' of God is to educate the aimless souls of 'Sheol' (the limbo between Heaven and Hell) so that they will be fit to enter the gates of Heaven and be redeemed. By doing this, it seems that he and God are working towards the same goal, but the difference (in the novel) is that God does not actually care about what happens to all of the lost souls of the dead or what happens on Earth in His name and Memnoch does.
Pen and ink.